Scheduler thrashing. This can happen under Unix when you have a
number of processes that are waiting on a single event. When that event (a
connection to the web server, say) happens, every process which could
possibly handle the event is awakened. In the end, only one of those
processes will actually be able to do the work, but, in the meantime, all
the others wake up and contend for CPU time before being put back to
sleep. Thus the system thrashes briefly while a herd of processes thunders
through. If this starts to happen many times per second, the performance
impact can be significant.